Internal parts of electrical power transformers and switchgear are normally immersed in an electrical insulating fluid. The electrical insulating fluid performs several functions and has specific requirements. It acts as an insulating medium between energized parts of the electrical equipment. It cools equipment by transferring heat from the windings and core of a transformer to a cooling surface. It quenches an arc created in the operation of switchgear or a fuse.
In case of accidental spills, the fluid should not be hazardous to animal or plant life. It should be biodegradable, meaning that microbes present in soil and water should be able to break down the chemical compounds of the fluid over time to substances that are less toxic, non-toxic, or inert. The fluid should be chemically stable during the useful life of the electrical equipment. It should exhibit low flammability, in case of a fire involving the electrical equipment.
Liquid insulating or dielectric fluids in electrical transformers and switchgear are well known in the art. See for example, U.S. Pat. Nos. 3,000,807; 3,095,366; 3,587,168; and 3,753,188. These patents describe the use of insulating fluids in equipment and the types of fluids so used. The most commonly used fluids have been petroleum oils, silicone fluids, or synthetic, hydrocarbon fluids. However, each of these materials has certain drawbacks, particularly with respect to environmental impact and biodegradability.
Silicone fluid, for example has been used widely as a fire-resistant insulating oil, but has been shown not to biodegrade to any appreciable extent. In addition, silicone fluid polymerizes when exposed to an electric arc, forming silicone gel particles that later interfere with the dielectric or insulating function of the fluid.
Petroleum fluids work well in most applications, but, depending on their chemistry, biodegrade very slowly. Spills of petroleum fluids have damaged soil and water ecosystems and can persist in the environment for years.
Petroleum, silicone, and synthetic hydrocarbon fluids thus perform well as insulating oils, but do not address the ever-increasing demands that fluids be more easily biodegraded and less harmful to the environment when spilled onto soil or water.
Due to lack of standardization of test methods and agreement on desired characteristics by industry, many levels of biodegradation exist. Petroleum products, for example, have been shown to biodegrade to a certain extent, if the proper conditions and species of bacteria are available in soil or water.
Because of their molecular structure, vegetable seed oils can be shown to biodegrade much more rapidly and completely than the above described electrical insulating fluids.
Vegetable seed oils have been tried as insulating oils in the past, but their use has been hindered by their poor stability. Vegetable seed oils age and become unstable at a much faster rate than petroleum or silicone products, especially at the elevated temperatures and in the presence of metal, typically found in electrical transformers.